I. Field of the Invention
The invention generally relates to mobile communication systems and in particular to techniques for compensating for frequency drift in a low frequency clock employed during a sleep period between paging slots within a mobile station of a mobile communications system.
II. Description of the Related Art
Certain state of the art wireless communication systems, such as Code Division Multiple Access (CDMA) Systems, employ slotted paging to allow mobile stations to conserve battery power. In a slotted paging system, paging signals are transmitted from a base station to particular mobile stations only within assigned paging slots separated by predetermined intervals of time. Accordingly, each individual mobile station may remain within a sleep mode during the period of time between consecutive paging slots without risk of missed paging signals. Whether any particular mobile station may switch from an active-mode to a sleep mode depends, however, upon whether the mobile station is currently engaged in any user activity such as processing input commands entered by the user or processing a telephonic communication on behalf of the user. Assuming though that the mobile station is not currently engaged in any processing on behalf of the user, the mobile station automatically powers down selected internal components during each period of time between consecutive slots. One example of a slotted paging system is disclosed in U.S. Pat. No. 5,392,287, entitled xe2x80x9cApparatus and Method for Reducing Power Consumption in a Mobile Receiverxe2x80x9d, issued Feb. 21, 1995, assigned to the assignee of the present invention and incorporated by reference herein.
Thus, within a slotted paging system, a mobile station reduces power consumption by disconnecting power from selected internal components during a sleep period between consecutive slots. However, even during the sleep period, the mobile station must reliably track the amount of elapsed time to determine when the next slot occurs to permit receive components of the mobile station to power up in time to receive any paging signals transmitted within the slot. One solution to this problem is to operate a high frequency clock throughout the sleep period and to track the amount of elapsed time using the high frequency clock. This solution allows the sleep period to be very precisely tracked using the high frequency clock. However, considerable power is consumed operating the high frequency clock and optimal power savings therefore are not achieved during the sleep period.
Hence, it would be desirable to instead employ an alternate low frequency, low power clock during the sleep period to further reduce power consumption. However, low frequency, low power clock signals typically suffer from considerable frequency drift such that the amount of elapsed time during the sleep period cannot be precisely determined. Frequency drift within a mobile station can be particularly significant as a result of temperature variations within the mobile station either as a result of changes in operation of components of the mobile station or as a result of ambient conditions of the mobile station. For example, during an extended telephone call, components of the mobile station may heat to 87 degrees Celsius. During an extended period of inactivity, the temperature of the components may cool to an ambient temperature of, perhaps, 25 degrees Celsius. Moreover, if the user places the mobile telephone in either a very hot or very cold location, the temperature change may be even more significant. Typical low power, low frequency clock signal generators are significantly affected by even relatively minor temperature changes and are even more strongly affected by such broad changes in temperature. Indeed, the amount of drift in a typical low power, low frequency clock signal is sufficiently great such that if used by itself to calculate the elapsed time, there is significant risk that the mobile station will not be reactivated in time to power up components to detect a paging signal transmitted within a next paging slot. Accordingly, important paging signals maybe missed possibly resulting in missed phone calls and the like.
Hence, when using a low-frequency clock signal to track time during a sleep period, the mobile station is typically configured to return to an active mode by activating a high frequency clock signal well in advance of a next expected paging slot to thereby avoid possible timing errors. Thus, for example, if the paging slots occur every 26.67 milliseconds, the mobile station may be programmed to activate the high frequency clock and to power up receive components after only, for example, 26 milliseconds of sleep to ensure that the next paging slot is not missed. Hence, optimal power savings are not achieved.
One technique that has been proposed for compensating for timing errors inherent in low frequency, low power clock signal generators is to adapt a length of a current sleep period based upon a timing accuracy of a previous sleep period. More specifically, if a previous sleep period was determined to be too long due to timing errors in the low power, low frequency clock generator, the mobile station is programmed to wake up earlier in the current sleep period. To determine whether a sleep period is too long or too short, the mobile station attempts to detect a unique word within a received paging signal, such as a message preamble which signifies the beginning of an assigned slot. If the unique word is not detected, the mobile station concludes that it woke up too late and therefore the sleep duration is decreased for subsequent sleep periods. If the unique word was properly received, the mobile station either woke up on time or wake up too early and the sleep duration is increased slightly for the subsequent sleep period. One problem with the aforementioned technique is that it assumes that any failure to detect the unique word is a result of a timing error. However, there may be other reasons besides the duration of the sleep period that the unique word was not correctly received and demodulated, such as poor communication channel quality conditions. Moreover, even if failure to detect the unique word was a result of a timing error rather than other communication errors, the system still does not precisely correct for errors in the low power, low frequency clock signal and therefore does not provide for optimal power savings.
A significant improvement is provided in U.S. patent application Ser. No. 09/134,808, entitled xe2x80x9cSynchronization of a Low Power Oscillator with a Reference Oscillator in a Wireless Communication Device Utilizing Slotted Pagingxe2x80x9d, filed Aug. 14, 1998 and assigned to the assignee of the present invention. In the aforementioned patent application, timing errors are corrected without relying upon the failure to receive portions of transmitted signals. Rather, the system includes a frequency error estimation unit for directly estimating the frequency of the low power, low frequency clock. In one example described in the patent application, the frequency error in the low frequency clock is determined by timing the low frequency clock using a high frequency clock during periods of time when the high frequency clock is active. For example, during each paging slot when the high frequency clock signal of the mobile station is activated, the frequency error in the low frequency clock is calculated based upon the high frequency clock. Additionally, the system operates to synchronize the activation of the high frequency clock very precisely to transitions in the low frequency clock signal to further reduce errors.
Although the system of the aforementioned patent application provides a significant improvement over systems which rely on the detection of unique words of signals transmitted to the mobile station, considerable room for improvement remains. To permit the mobile station to respond promptly to any keys that have been pressed by a user during a sleep period, it is often desirable to subdivide the sleep period into a sequence of sub-periods, also referred to herein as xe2x80x9ccatnapsxe2x80x9d. After each catnap, selected components of the mobile station are powered up sufficiently to detect whether a key on the keypad has been pressed and, if so, the sleep period is aborted and other components of the mobile station are powered up as needed to respond to the pressed key. The duration of the catnaps are typically not an interger number of cycles of the low frequency sleep mode clock. Accordingly, considerable truncation errors can occur if the low frequency clock, by itself, is employed to time the catnaps. Hence, it would be desirable to provide a system for timing sleep periods using a low frequency clock in such a manner to eliminate substantial truncation errors and aspects of the invention are directed to this end. Also, because the frequency error is calculated only while the mobile station is in an active-mode, it may not properly detect frequency errors occurring during extended sleep periods during which time the temperature of the low frequency clock signal generator decreases significantly. Accordingly, even with the improved system of the patent application, the high frequency clock signal must be usually be activated somewhat in advance of the next expected paging slot to account for remaining timing errors. Hence, optimal power savings are not achieved. It would be also preferable to provide a system wherein frequency drift is estimated effectively to permit an active mode high frequency clock to be turned on as close as possible to the next paging slot to permit maximum power savings during the sleep period and to permit easy reacquisition of a paging signal and it is to these ends that other aspects of the present invention are also directed.
In accordance with a first aspect of the invention, a method is provided for tracking the length of a sleep period within a mobile station using a sleep clock. The method operates to precisely calibrate portions of the sleep period. In accordance with the method, a sleep period is initiated with the sleep period subdivided into a sequence of sub-periods each of known duration but wherein the durations of the sub-periods are not necessarily integer multiples of cycles of the sleep clock. Elapsed time is tracked within each individual sub-period of the sleep period using an integer sleep counter which tracks whole cycles of the sleep clock. Any remaining fractional portions of the cycles of the sleep mode clock not accounted for by the integer sleep counter are tracked using a fractional sleep counter, with the fractional sleep counter accumulating remaining fractional portions of sleep clock cycles from one sub-period to the next.
In an exemplary embodiment of the first aspect of the invention, the sub-periods of the sleep period are xe2x80x9ccatnapsxe2x80x9d. Within each catnap, the integer sleep counter is incremented downwardly on each cycle of the sleep clock. When the integer sleep counter reaches 0, the catnap is deemed to be complete. When the catnap is complete, a keypad of the mobile station is checked to determine whether a key has been pressed and, if so, the sleep period is terminated. Whenever the fractional counter overflows, a current value of the integer sleep counter is increased by a integer overflow portion of the fractional sleep counter such that the integer counter then accounts for the overflow. A current value of the fractional sleep counter is reset to be equal only to the remaining fractional portion, if any, of the previous fractional sleep counter value such that the fractional sleep counter continues to track remaining fractional portions of cycles of the sleep mode clock.
In accordance with a second aspect of the invention, a method is provided for compensating for frequency drift within a sleep clock signal used to time sleep periods during a slotted paging mode of operation of a wireless mobile station wherein the wireless mobile station receives signals from a base station having high timing accuracy. The method operates to iteratively adjust an estimate of the frequency drift during a sleep mode to enable effective frequency drift compensation. In accordance with the method, an initial frequency of the sleep clock signal is determined following power-up of the mobile station. A fixed frequency drift compensation factor representative of a difference between the initial frequency of the sleep clock signal and a predetermined nominal frequency (that eliminates truncation error) is then determined for computational convenience. A dynamic frequency error compensation factor representative of a difference between the initial frequency and a current dynamic frequency of the slow clock signal (which may vary due to temperature or aging) is estimated. Then, during the slotted mode of operation, the following steps are iteratively performed. The dynamic frequency error compensation factor is updated by determining an amount of timing slew between the mobile station and the base station, and then determining new values for the dynamic frequency compensation factor by applying a value representative of the amount of the slew to a feedback loop configured to provide a new dynamic frequency error compensation factor having a value selected to achieve a subsequent reduction in slew.
In an exemplary implementation, the sleep period length is converted into the number of sleep clocks using the dynamic frequency as the initial estimate. After each wakeup from a sleep period, the mobile station searches for an incoming signal from the base station. As timing is maintained at the base station with very high accuracy, any error made in the initial estimate of dynamic frequency (arising due to truncation effects or temperature-and aging-induced frequency drifts) will show up as a xe2x80x9cslewxe2x80x9d in the timing of the incoming signal. The quantity xe2x80x9cslewxe2x80x9d indicates the timing difference or offset that the mobile perceives after wakeup from sleep. Then a new value for the dynamic frequency compensation factor is determined by applying a value representative of the amount of the slew to a loop filter.
In a preferred implementation, the mobile station is configured to implement both the improved calibration method and the improved frequency drift estimation method. Apparatus embodiments of the invention are also provided.